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I have this exact motherboard and even ran a Cel 600 in it for awhile! The first menu at the top of the first column (forget what it's called) will be the one you want. Set your processor speed to "User Define", and then set your Front Side Bus to 100. That should get your Cel 600 at 900. Also, make sure that "Speed Error Hold" is off because I'm not sure if it defaults to "on" or not.